1. Field of the Invention
This invention relates to LED-based (Light Emitting Diode-based) light emitting systems and devices with color compensation.
2. Description of the Related Art
White light emitting LEDs (“white LEDs”) are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor combined with the light emitted by the phosphor provides light which appears to the human eye as being white in color.
Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (70 lumens per watt and higher) high brightness white LEDs are increasingly being used to replace conventional fluorescent, compact fluorescent and incandescent light sources.
The ability of a light source to render the color of an object is measured using the Color Rendering Index (CRI) which gives a measure of how a light source makes the color of an object appear to the human eye and how well subtle variations in color shade are revealed. CRI is a relative measurement of the light source's ability to render color compared with a black body radiator. In applications where accurate color rendition is required, such as for example retail lighting, museum lighting and lighting of artwork, a high CRI (typically at least 90) is highly desirable.
A disadvantage of white LEDs can be their relatively low CRI, typically <75, compared with an incandescent source whose CRI is >95. The low CRI is due to the absence of light in the red (>600 nm) part of the spectrum. To improve the CRI of a white LED it is known to incorporate a red emitting LED. U.S. Pat. Nos. 6,513,949 and 6,692,136, both to Marshall et al., teach hybrid white LED lighting systems comprising a combination of one or more LEDs (red or green) and a phosphor-LED consisting of a blue LED and at least one phosphor (green or amber).
U.S. Pat. No. 6,577,073 to Shimizu et al. disclose an LED lamp that includes blue and red LEDs and a phosphor. The blue LED produces an emission falling within a blue wavelength range. The red LED produces an emission falling within a red wavelength range. The phosphor is photo-excited by the emission of the blue LED to exhibit photoluminescence having an emission spectrum in an intermediate wavelength range between the blue and red wavelength ranges.
U.S. Pat. No. 7,213,940 to Van Den Ven et al. disclose a white light emitting device that comprises first and second groups of solid state light emitters (LEDs) which emit light having a dominant wavelength in a range 430 nm to 480 nm (blue) and 600 nm to 630 nm (red) and a phosphor material which emits light with a dominant wavelength in a range 555 nm to 585 nm (yellow).
In lighting applications it is important to have color control to maintain the same CCT (Correlated Color Temperature) over the life of the LED lighting system. Several factors can contribute to a color change in LED-based light emitting devices. These factors include aging of the LED die and/or phosphor, operating temperature and aging of electronic drive components.
Whilst the use of red LEDs combined with blue LEDs and phosphors to create white light has shown advantages for creating high CRI light, high R9 content and high efficiency warm light, one problem with this type of device is that the red LED typically ages faster than the blue LEDs and the device's emission product, most notably CCT and CRI, will change with both operating time and temperature. This effect is called differential aging and it results in a color shift of the light over time. For many lighting applications such a color shift is unacceptable and causes problems such as for example old fixtures no longer matching the light color of new fixtures and lighting that falls out of specification. As represented in FIG. 1a the changes in emission intensity of blue and red light emitting LEDs with operating temperature and time are different. Typically the emission intensity of a red LED decreases significantly quicker than a blue LED with increased operating temperature and time. For example over an operating temperature range of 25° C. to 75° C. the emission intensity of a GaN-based blue LED can decrease by about 5% whilst the emission intensity of a AlGaInP-based red LED can decrease by about 40%. In a white light emitting device based on blue and red LEDs these different emission intensity/time and/or emission intensity/temperature characteristics will, as shown in FIG. 1b, result in a change in the spectral composition of the emission product and an increase in CCT with increased operating time and temperature. Moreover as shown in FIG. 1b a reduction in the relative proportion of red light in the emission product with increasing operating temperature and time will result in a decrease in CRI.
Colorimeters are well understood and it is known to integrate a colorimeter into lighting systems. Such systems typically incorporate three or more photo sensors that are color sensitive (RGB for example). The calibration and accuracy of colorimetric systems can be challenging and expensive. Shifts in performance of the color sensors can introduce color error to the systems so these devices need to be very accurate and well calibrated. For many applications, such as for example general lighting, colorimetric systems are prohibitively expensive.
It is an object of the present invention to provide a light emitting system and/or device that in part at least overcomes the limitations of the known devices.